CN113390899A

CN113390899A - Microwave reflectometer with online automatic calibration function

Info

Publication number: CN113390899A
Application number: CN202110607222.XA
Authority: CN
Inventors: 文斐; 张涛; 李恭顺; 高翔; 韩翔; 叶凯萱; 吴茗甫; 黄佳; 耿康宁; 周振; 钟富彬; 刘煜凯; 向皓明
Original assignee: Hefei Institutes of Physical Science of CAS
Current assignee: Hefei Institutes of Physical Science of CAS
Priority date: 2021-06-01
Filing date: 2021-06-01
Publication date: 2021-09-14
Anticipated expiration: 2041-06-01
Also published as: CN113390899B

Abstract

The invention relates to a microwave reflectometer with on-line automatic calibration function, comprising a microwave reflectometer body and a calibration component. Wherein, the second data acquisition module is configured to obtain a dynamic working curve formed by the working voltage of the swept-frequency microwave source, and the time-to-digital converter is configured to measure the first time delay between the trigger signal and the swept-frequency microwave signal, and the trigger signal a second time delay from the beat signal, the first data acquisition module is configured to measure a third time delay between the reference signal and the reflected signal, the controller is configured to be connected to the arbitrary waveform generator (102), and Using the dynamic working curve and correcting the sweep frequency control voltage output by the arbitrary waveform generator (102) based on the first, second and third time delays, the beat signal is corrected and the calibration accuracy is improved.

Description

Microwave reflectometer with online automatic calibration function

Technical Field

The invention belongs to the technical field of plasma diagnosis, particularly relates to a microwave reflectometer with an online automatic calibration function, and belongs to the technical field of microwave diagnosis.

Background

Microwave reflectometers are a tool for measuring plasma density distribution, often used on a variety of fusion devices. The microwave reflectometer most commonly used at present is a continuous wave frequency modulation method, which works by generating a microwave signal with continuously changing frequency by a sweep frequency microwave source and transmitting the signal to plasma. Because the reflection cross sections of the microwave signals with different frequencies in the plasma are related to the plasma density, the positions of different reflection cross sections can be obtained by calculating the flight time of different microwave signals in the plasma, so that the positions of different plasma densities, namely the plasma density distribution, are corresponded. Due to the limited transmission distance, the total time of flight of the microwave signal is usually in the order of nanoseconds, and it is relatively difficult to measure the time of flight directly, and the transmitting signal and the receiving signal are usually mixed and filtered to obtain a beat signal. By measuring the frequency of the beat frequency signal and combining the sweep frequency speed, the flight time of the microwave signal can be calculated.

In continuous wave frequency modulated microwave reflectometers, the emission measurement must be of a certain accuracy, otherwise the plasma density distribution calculation will be subject to large errors, which must be avoided in an effort. The following aspects are mainly involved in causing errors in the emission measurement:

first, the output of the microwave source is nonlinear. Due to the nonlinearity existing between the input voltage and the output frequency of the microwave source, the output frequency has errors.

Secondly, the transmission line has dispersion characteristics. In a transmission line, particularly a waveguide, microwaves of different modes and frequencies have different group velocities, resulting in dispersion in the microwave transmission, thereby causing a shift in beat frequency.

Third, frequency drift due to temperature and aging. The variation of the operating characteristics of the electronic device due to the variation of the operating temperature and the extension of the operating time deviates from the original calibration value, and measurement errors are generated.

The calibration of the transmitting frequency of the microwave reflectometer is mainly carried out by the following steps:

firstly, a control signal with a constant voltage is input to the microwave source, and the output frequency of the microwave source is measured. And then increasing the voltage of the control signal according to a certain step value, and respectively measuring the output frequency of the control signal to obtain a voltage-frequency relation curve of the microwave source. And performing piecewise linear fitting on the voltage-frequency relation curve, calculating corresponding working voltage according to the equal frequency interval, and finally generating a sweep frequency control voltage curve. This step mainly corrects for the non-linearity of the microwave source.

And secondly, enabling the microwave reflectometer to aim at a metal plane with a fixed position for frequency sweeping emission. Since the sweep rate is constant, ideally the frequency of the beat signal of the microwave reflectometer should be a constant value, but for the various reasons mentioned above, the beat frequency will drift. And adjusting the sweep frequency control voltage curve by measuring the error between the actual beat frequency and the ideal value until a beat frequency signal with smaller error is generated.

In the conventional method, the delay time of the control signal and the microwave signal in the transmission line system is not considered, so that a certain phase error actually exists in the calibration data in the calibration process. Under the condition of slow sweep frequency speed, the influence of the phase error is not obvious, but when the sweep frequency speed is increased to 1MHz, because the frequency change speed is too fast, the influence of several MHz or even tens of MHz is brought to the beat frequency by the tiny phase error, the calibration result is influenced, and even the calibration fails.

In order to ensure the calibration accuracy, the conventional calibration is usually performed in a laboratory and is realized by manual offline operation, which has the disadvantages that the laboratory test environment is deviated from the field environment of the device, the obtained correction result may not be adapted in a new working environment, and a new error is generated. Moreover, the calibration process involves the disassembly and assembly of the microwave reflectometer apparatus, which is cumbersome, so that the calibration cycle is usually long, sometimes up to several months, during which the operating state of the apparatus is likely to have changed, resulting in errors in the measurement results.

Disclosure of Invention

The technical problem to be solved by the invention is as follows: aiming at a microwave reflectometer of a continuous wave frequency modulation mode, the defects of the prior art are improved, and the microwave reflectometer with an online automatic calibration function is provided. The calibration component corrects the sweep control voltage based on various delay times of signals in the transmission line system, so that the calibration accuracy is improved.

The technical problem solved by the invention can be realized by the following technical scheme:

a microwave reflectometer with on-line automatic calibration function comprises a microwave reflectometer body, wherein the microwave reflectometer body comprises a sweep frequency microwave source, an arbitrary waveform generator, a first directional coupler, a local vibration source, a single-side band modulator, a power divider, a first frequency mixer, a first in-phase quadrature demodulator, a first data acquisition module and a controller, the frequency sweeping microwave source generates frequency sweeping microwave signals under the control of frequency sweeping control voltage signals generated by an arbitrary waveform generator, the local vibration source generates baseband signals with fixed frequency, the power divider divides the baseband signals into multiple paths to be output, the first directional coupler divides the frequency sweeping microwave signals into two parts, namely detection signals and reference signals, the single-sideband modulator synthesizes the baseband signals and the detection signals to generate upper sideband signals, the upper sideband signals are transmitted to plasma, and the receiving antenna receives reflected signals reflected from a plasma cut-off layer; the first mixer mixes the reference signal with a reflected signal to obtain an intermediate frequency signal, the first in-phase quadrature demodulator performs complex mixing on the intermediate frequency signal and a baseband signal to obtain a beat frequency signal, and the calibration device is characterized by further comprising a calibration component, the calibration component comprises a second directional coupler, a frequency synthesis source, a second mixer, a second data acquisition module, a time-to-digital converter and a clock synchronization module, wherein the second directional coupler separates a part of a frequency sweep microwave signal output by a frequency sweep microwave source to serve as a calibration signal, the frequency synthesis source generates a microwave signal with a fixed frequency, the second mixer is used for mixing the calibration signal and the microwave signal with the fixed frequency to generate a difference frequency signal of the calibration signal and the microwave signal with the fixed frequency, and the second data acquisition module is configured to obtain a dynamic sweep frequency working curve formed by microwave working voltages of the frequency sweep source based on the difference frequency signal, the clock synchronization module is controlled by the controller to generate a trigger signal, the time-to-digital converter is configured to measure a first time delay between the trigger signal and a swept frequency microwave signal and a second time delay between the trigger signal and a beat frequency signal, the first data acquisition module is configured to measure a third time delay between a reference signal and a reflected signal, and the controller is configured to be connected with the arbitrary waveform generator, and correct a sweep frequency control voltage output by the arbitrary waveform generator based on the first, second and third time delays by using the dynamic working curve, so as to correct the beat frequency signal.

The frequency sweep microwave source generates frequency sweep microwave signals with frequency changing along with voltage under the control of frequency sweep control voltage signals generated by the arbitrary waveform generator.

The microwave reflectometer body further comprises a first frequency multiplier, a second frequency multiplier and a third frequency multiplier, wherein the first frequency multiplier is used for multiplying the frequency of the upper sideband signal, the second frequency multiplier is used for multiplying the frequency of the reference signal, the third frequency multiplier is used for multiplying the frequency of the baseband signal, the upper sideband signal after frequency multiplication is transmitted to the plasma, the reference signal output by the second frequency multiplier and the reflection signal are mixed by the first frequency mixer, and the first in-phase quadrature demodulator carries out complex mixing on the intermediate frequency signal and the baseband signal output by the third frequency multiplier.

The calibration component further comprises a programmable delay line, the programmable delay line is used for adjusting the delay time of the reference signal output by the first directional coupler, and the two microwave switches are switched to realize the switching of coaxial cables with different lengths.

The first data acquisition module acquires the same-direction component, the orthogonal component and the sweep frequency control voltage signal and outputs the same-direction component, the orthogonal component and the sweep frequency control voltage signal to the controller.

The calibration component further comprises a narrow-band filter and a second in-phase and quadrature demodulator, the second mixer outputs signals to the narrow-band filter, the center frequency of a pass band of the narrow-band filter is equal to the frequency generated by the vibration source, the second in-phase and quadrature demodulator is used for extracting an in-phase component and a quadrature component of signals output by the narrow-band filter, and the second data acquisition module is used for digitizing the signals output by the second in-phase and quadrature demodulator and transmitting the signals to the controller.

The core component of the sweep frequency microwave source is a voltage-controlled oscillator.

After the microwave reflectometer is installed on the fusion device, the calibration operation can be carried out in the running clearance of the device. Because no plasma exists in the device, the microwave signal is directly reflected by the inner wall of the device after being transmitted, the measurement result is relatively constant, and the calibration operation is carried out under the environment. The online automatic calibration method of the microwave reflectometer comprises the following steps:

measuring time delay between a trigger signal and a sweep frequency microwave signal and a beat frequency signal. Generating a height V by the controller₁Width of T₁The pulse waveform of (a) is output to an arbitrary waveform generator, from which it is loaded into a memory. The controller instructs the clock synchronization module to generate a trigger signal and output the trigger signal to the arbitrary waveform generator and the time-to-digital converter. At willThe waveform generator outputs the pulse waveform in the memory immediately after receiving the trigger signal. Meanwhile, the time-to-digital converter starts timing by taking the trigger signal as a start signal and stops timing by taking the pulse signal output by the arbitrary waveform generator as a stop signal in the channel 1. In the channel 2, the time-to-digital converter starts timing by taking the trigger signal as a start signal, and stops timing by taking an in-phase signal output by the first in-phase quadrature demodulator as a stop signal after being discriminated. Timing result T obtained by time-to-digital converter in channel 1_{D_SWEEP}I.e. the time delay between the trigger signal and the swept frequency signal, the timing result T obtained in channel 2_{D_BEAT}I.e. the time delay between the trigger signal and the beat signal. In order to reduce errors, the above measurement process needs to be repeated for many times, and the average result is obtained.

And step two, measuring the time delay between the reference signal and the reflected signal. In general, to reduce the intermediate frequency, the time delay between the reference signal and the reflected signal needs to be reduced, so that the reference signal needs to reach the mixer through a delay line, but too small a time delay causes great difficulty in measurement. For calibration purposes, the delay line is modified to be a programmable delay line that can be programmed to switch between a long coaxial cable and a short coaxial cable, which is controlled by a controller. In this step, the programmable delay line is instructed by the controller to switch from a long coaxial cable to a short coaxial cable, thereby increasing the time delay between the reference signal and the reflected signal. Generating a height V by the controller₂Width of T₂The narrow pulse waveform is output to an arbitrary waveform generator from which it is loaded into memory, where T₂Should be less than the estimated time delay between the transmitted and reflected signals. The controller instructs the clock synchronization module to generate a trigger signal and output the trigger signal to the arbitrary waveform generator and the first data acquisition module. The random waveform generator immediately outputs the narrow pulse waveform after receiving the trigger signal, and simultaneously the first data acquisition module starts to acquire data. The reference signal firstly reaches the mixer through a delay line, and the reflected signal is transmitted through a waveguide and internally reflected by the device and then reaches the mixer. After mixing, two clusters of waveforms are generated on the beat signal. By a first numberAfter the beat frequency signal is collected by the collecting module, the upper envelope of the signal is extracted and the peak searching calculation is carried out, and the time interval T between two peak values_{D_TOF_NL}I.e. the time delay between the reference signal and the reflected signal. In order to reduce errors, the above measurement process needs to be repeated for many times, and the average result is obtained. Finally, the controller instructs the programmable delay line to switch from the short coaxial cable back to the long coaxial cable. The time delay between the reference signal and the reflected signal is also given by T_{D_TOF_NL}Minus the delay T caused by the difference in cable length_LINEObtained, i.e. T_{D_TOF}＝T_{D_TOF_NL}-T_LINE。

And step three, measuring the dynamic working curve of the microwave source. The controller generates a sweep from 0V to V_FSIs transmitted to and stored by an arbitrary waveform generator, wherein V_FSCorresponding to the control voltage corresponding to the highest output frequency of the swept frequency microwave source. The controller instructs the frequency synthesis source to output a fixed frequency microwave signal. The controller instructs the clock synchronization module to generate a trigger signal and output the trigger signal to the arbitrary waveform generator and the second data acquisition module. The random waveform generator outputs linear scanning waveform immediately after receiving the trigger signal. The frequency of the output of the frequency sweeping microwave source is F due to the second mixer_PFrequency F of frequency sweep signal and frequency synthesis source output of (T)_FIXWhile the output is provided with a narrow-band filter, so that only when the mixer output frequency falls within the pass band (F)_BC-ΔB/2≤|F_P(T)-F_FIX|≤F_BCSignals within + Δ B/2) can be acquired by the second data acquisition module. After the signal passes through a second in-phase and quadrature demodulator, the in-phase and quadrature components of the signal are extracted. The second data acquisition module is responsible for acquiring the in-phase component I (T), the quadrature component Q (T) and the linear scanning signal V (T). After the in-phase component and the orthogonal component are synthesized into a complex signal, the amplitude value of the complex signal is extracted to generate an amplitude signal A (T). And (D) obtaining the time of the two peaks by utilizing a peak searching algorithm when the two peaks exist in A (T). Querying a linear scanning signal V (T), wherein the scanning voltages corresponding to the moments of the two peak values are V respectively_S1And V_S2。V_S1Corresponds to frequency F_P(T)＝F_FIX-F_BC，V_S2Corresponds to frequency F_P(T)＝F_FIX+F_BCThus, the center point voltage V between the two voltages_S＝(V_S1+V_S2) A/2 corresponds to the frequency F_FIX. From this, the output frequency of the sweep frequency microwave source is F_FIXThe working voltage is V_S. In order to reduce errors, the measurement process needs to be repeated for multiple times, and an average value is taken as a measurement result. At the completion frequency F_FIXAfter the corresponding working voltage is measured, the controller controls the frequency synthesis source to convert F_SAnd repeating the steps according to a certain increment value until the working voltage measurement in the whole output frequency range of the frequency sweeping microwave source is completed, thereby obtaining the dynamic working curve of the frequency sweeping microwave source.

And step four, calibrating the beat frequency signal. The controller utilizes the dynamic working curve of the sweep frequency microwave source obtained in the step three and according to the sweep frequency period T_SWEEPGenerating a sweep frequency control waveform W at equal frequency intervals_SWEEPWherein the number n of points of the waveform is determined by the sweep period T_SWEEPAnd data rate F of an arbitrary waveform generator_DACDetermining, i.e. n ═ T_SWEEP*F_DAC. The controller will W_SWEEPSent to and stored by an arbitrary waveform generator. The controller instructs the clock synchronization module to generate a trigger signal and output the trigger signal to the arbitrary waveform generator and the first data acquisition module. The arbitrary waveform generator receives the trigger signal and immediately converts W_SWEEPAnd (6) outputting. The first data acquisition module immediately starts to acquire a sweep frequency control voltage signal output by an arbitrary waveform generator and a beat frequency signal output by an in-phase quadrature demodulator after receiving a trigger signal, and acquires the time length T_SAMPLEShould be greater than or equal to T_{D_BEAT}+T_{D_TOF}+T_SWEEPThus, the complete beat frequency signal can be ensured to be recorded. Recording the collected sweep frequency control voltage signal as S_SWEEPAnd the collected beat frequency signal is recorded as S_BEATThe number of sampling points is determined by the sampling duration T_SAMPLEAnd the sampling rate F of the first data acquisition module_ADCDetermination, i.e. T_SAMPLE*F_ADC. To align dataAnalysis, first, S_SWEEPHas a front face length of T_{D_SWEEP}*F_ADCThe data of (1) is cut off, and then the front M is taken as T in the rest data_SWEEP*F_ADCPoints, the data thus intercepted being denoted S_{SWEEP_S}All information of the sweep control voltage is stored in the data. The beat signal is obtained by mixing the reference signal and the reflected signal, due to the time delay T between the two signals_{D_TOF}Resulting in the length of the beat signal being longer than the sweep period T_SWEEPIncreasing a period of time T_{D_TOF}. For the whole beat frequency signal, the previous period is T_{D_TOF}Is obtained by mixing a reference signal with a reflected constant frequency signal, and the last period of time is T_{D_TOF}The signals are mixed by a reference signal which becomes a fixed frequency and a reflected sweep frequency signal, and the signals do not have the practical significance of calibration, so that the two signals cannot be used in the calibration process. To align with the swept control voltage signal, S_BEATMiddle front T_{D_BEAT}*F_ADCData is truncated, and then in the remaining signal, M is truncated to T_SWEEP*F_ADCPoints, the data thus intercepted being denoted S_{BEAT_S}. At S_{BEAT_S}In the method, a front section of the beat frequency signal is reserved with the length of T_{D_TOF}Is truncated by a subsequent segment of length T_{D_TOF}Is performed to facilitate alignment with the swept control voltage signal, facilitating analysis. To S_{BEAT_S}Performing fast Fourier transform, windowing, window width w and step length 1 to obtain time frequency spectrum FS_{BEAT_S}. Will S_{SWEEP_S}Copy one copy, denoted S_{SWEEP_FIX}And the frequency sweep control waveform is used for storing the corrected frequency sweep control waveform. FS (file system)_{BEAT_S}And S_{SWEEP_S}The number of data points in (1) is M, and the calculation is started from the Mth data, and the calculation is carried out on the FS_{BEAT_S}(M) performing peak searching to obtain the maximum component in the frequency spectrum corresponding to the frequency F_BAnd (M) is the beat frequency. The beat frequency is actually mixed by the reference signal and the reflected signal at the time of M point, and the frequency of the reference signal at the M point is controlled by the sweep control voltage S_{SWEEP_S}(M) determining, and reflectingThe signal frequency is composed of_{D_TOF}Previous sweep control voltage S_{SWEEP_S}(M- Δ M), where Δ M ═ T_{D_TOF}*F_ADC. Since the reflected signal is transmitted through the waveguide to generate dispersion, and therefore frequency drift occurs, we need to modify S_{SWEEP_S}(M- Δ M) to correct the beat frequency. The specific correction process is as follows. The voltage S will now be controlled according to the corrected frequency sweep_{SWEEP_FIX}(M) and a microwave source frequency sweep control curve, and the ideal frequency of the reference signal at the M point can be calculated to be F_{P_LO_FIX}(M) recombining the ideal beat frequencies F_{B_FIX}＝(dF_P/dt)*T_{D_TOF}The ideal frequency at which the reflected signal is obtained should be F_{P_RF_FIX}(M)＝F_{P_LO_FIX}(M)-F_{B_FIX}. Controlling the voltage S according to the frequency sweep_SWEEP(M) and a microwave source frequency sweep control curve, and the actual frequency of the reference signal at the M point can be calculated to be F_{P_LO}(M) recombining the actual beat frequencies F_BThe actual frequency of the reflected signal is obtained as F_{P_RF}(M)＝F_{P_LO}(M)-F_B. The difference between the ideal frequency and the actual frequency of the reflected signal is F_{P_RF_FIX}(M)-F_{P_RF}(M). Since the difference is via T_{D_TOF}Cumulatively, so that the correction frequency is uniformly distributed to Δ m sweep points to obtain a correction coefficient: eta is 1+ [ F ]_{P_RF_FIX}(M)-F_{P_RF}(M)]/(F_{P_RF}(M) Δ M. Controlling the voltage S according to the frequency sweep_{SWEEP_S}(M-Deltam) and microwave source frequency sweep control curve to obtain the original detection frequency F_P(M- Δ M) multiplied by a correction coefficient η to obtain a corrected frequency F_{P_FIX}(M-Δm)＝η*F_P(M- Δ M), re-inquiring the microwave source sweep control curve, and comparing F_{P_FIX}(M- Δ M) is converted to a voltage value and filled with the corrected sweep control voltage S_{SWEEP_FIX}(M-. DELTA.m). And finishing the beat frequency correction at the Mth point, and correcting the M-1 and the M-2 one by the analogy until the point of the Deltam + 1. Ending at Δ m +1 because at the Δ m +1 th point, S is actually corrected_{SWEEP_FIX}(1) And the correction is finished. Finally the controller will obtain S_{SWEEP_FIX}Writing arbitrary waveformsThe generator and stored by it. Since the effect of a single calibration may not meet the requirement, the step needs to be repeated for multiple times until the beat frequency error of each detection point is smaller than the threshold value. All correction steps are now complete.

The invention has the following beneficial effects:

in the calibration process, various delays of signals in a transmission line system are fully considered, calibration data are accurately aligned, and the calibration accuracy is improved.

The calibration structure provided by the invention can automatically complete the calibration process by programming the controller, and no circuit connection switching exists in the process, so that manual intervention is not needed.

The calibration assembly is completely integrated into the original system of the microwave reflectometer and forms a whole with the reflectometer, and the detection performance of the microwave reflectometer is not influenced by the calibration assembly.

Drawings

In order to make the objects, technical solutions and advantages of the present invention clearer, the present invention is described in detail with reference to the accompanying drawings.

Fig. 1 is an overall structure diagram of a microwave reflectometer with an online automatic calibration function according to the present invention.

Detailed Description

Embodiments of the present invention will be described below with reference to the accompanying drawings. It is to be understood that such description is merely illustrative and not intended to limit the scope of the present invention. Embodiments of the present invention are described below with reference to a specific microwave reflectometer apparatus and calibration procedure.

Fig. 1 is an overall structure diagram of a microwave reflectometer with an online automatic calibration function according to the present invention. The microwave reflectometer with the online automatic calibration function is additionally provided with a calibration component on the basis of a microwave reflectometer body. The microwave reflectometer body is a typical continuous wave frequency modulation working mode, and specifically comprises a sweep frequency microwave source 101, an arbitrary waveform generator 102, a first directional coupler 103, a local oscillation source 104, a single-sideband modulator 105, a power divider 106, a first frequency multiplier 107, a second frequency multiplier 108, a third frequency multiplier 109,the system comprises a power amplifier 110, a first mixer 111, a first in-phase quadrature demodulator 112, a transmitting antenna 113, a receiving antenna 114, a first data acquisition module 115 and a controller 116. The working mode of the microwave reflectometer body is as follows: the core component of the sweep frequency microwave source 101 is a voltage-controlled oscillator, and a sweep frequency microwave signal with frequency changing along with voltage is generated under the control of a sweep frequency control voltage signal generated by an arbitrary waveform generator 102, and the emission frequency of the sweep frequency microwave signal is F_P(t) of (d). The local oscillator 104 generates a baseband signal having a fixed frequency F_L. The power divider 106 is configured to divide the local oscillator signal into multiple outputs. The first directional coupler 103 is used for dividing a microwave signal generated by the sweep frequency microwave source into two parts, wherein most of power is called a detection signal and is used for transmitting to plasma, and a small part of power is called a reference signal and is used for mixing with an echo signal to realize heterodyne measurement. The single sideband modulator 105 is used to convert a frequency of F_LThe base band signal is synthesized to the sweep frequency microwave signal to generate the frequency F_P(t)+F_LThe upper sideband signal of (a). The frequency multiplier is arranged for multiplying the frequency of the microwave signal by a certain factor M, and the first frequency multiplier 107 is arranged for multiplying the frequency F of the upper sideband signal_P(t)+F_LFrequency doubling to frequency M F_P(t)+M*F_LThe second frequency multiplier 108 is used for multiplying the reference signal frequency F_P(t) multiplication to M x F_P(t), the third frequency multiplier 109 is used for multiplying the local oscillator signal frequency F_LFrequency doubling to M x F_L. The power amplifier 110 is used to power amplify the microwave signal for transmission. The transmitting antenna 113 is used to transmit a detection signal to the plasma. When the detection signal is transmitted to the plasma stop layer, reflection is generated. The receiving antenna 114 is used to receive the reflected signal reflected back from the plasma stop layer. The first mixer 111 is used to mix the reference signal with the reflected signal, and the frequency of the reflected signal is changed to M × F due to transmission_P(t+Δt)+M*F_LAnd the frequency of the reference signal is M x F_P(t), then mixing to obtain the frequency M F_P(t+Δt)+M*F_L-M*F_P(t) intermediate frequency signal. The first in-phase quadrature demodulator 112 is used to further complex mix the intermediate frequency signal with the baseband signalMixing to obtain frequency F_B＝M*F_P(t+Δt)-M*F_P(t) beat frequency signal. After complex mixing, the output beat frequency signal has an in-phase component and a quadrature component, which is convenient for further time-frequency analysis and reduces noise caused by signal aliasing. The first data acquisition module 115 acquires the in-phase quadrature signal output by the first in-phase quadrature demodulator 112 and the voltage control signal generated by the arbitrary waveform generator 102, and outputs the signals to the controller 116.

The calibration component comprises a second directional coupler 201, a frequency synthesis source 202, a second mixer 203, a narrow-band filter 204, a second in-phase and quadrature demodulator 205, a second data acquisition module 206, a time-to-digital converter 207, a programmable delay line 208 and a clock synchronization module 209. The second directional coupler 201 is mainly used for outputting frequency F from the sweep frequency microwave source_PAnd (t) separating a small part of power from the signal as a calibration signal. The frequency synthesis source 202 is used to generate a fixed frequency F_SThe microwave signal of (2). The second mixer 203 is used for converting the frequency into F_P(t) calibration signal and frequency F_SIs mixed to generate a difference signal with a frequency of | F_P(t)-F_SL. The passband of the narrow band filter 204 has a center frequency F_BCThe pass band width is Δ B. To facilitate in-phase and quadrature demodulation using the signal of the local oscillator 104, F_BCShould correspond to the frequency F of the local oscillator 104_LAre equal. Only when the frequency of the difference frequency signal output by the second mixer 203 satisfies | F_P(t)-F_SI is greater than or equal to F_BC- Δ B/2, less than or equal to F_BCAnd + Δ B/2 is allowed to pass. The second in-phase and quadrature demodulator 205 is used to extract the in-phase component and the quadrature component of the signal. The second data acquisition module 206 is configured to digitize the signal output by the narrow band filter 204 and transmit the digitized signal to the controller 116. The time-to-digital converter 207 is used to measure the time delay between the trigger signal and the frequency sweep signal and beat signal. The programmable delay line 208 is used to adjust the delay time of the reference signal, which is switched by two microwave switches to adjust coaxial cables with different lengths. The clock synchronization module 209 is used for the arbitrary waveform generator 102 and the first data acquisition module 115The trigger signal and the clock signal are provided so that the signals and data can be aligned on the time axis.

In order to ensure the work synchronization, the clock synchronization module 209 is connected with the arbitrary waveform generator 102, the second data acquisition module 206, the time-to-digital converter 207, and the second data acquisition module 115 by using coaxial cables of the same length. The arbitrary waveform generator 102 is connected to the swept microwave source 101 and the data acquisition module 206 using a common length of coaxial cable.

In order to improve the system integration level, the second data acquisition module 206, the time-to-digital converter 207, the clock synchronization module 209 and the arbitrary waveform generator 102 are integrated on a printed circuit board, so that the space occupation of the calibration device on the microwave reflectometer system can be reduced. The circuit board takes a field programmable logic array (FPGA) chip as a core and combines a peripheral chip to realize the functions of the module. Specifically, the second data acquisition module 206 is composed of an off-chip amplification filter circuit, an analog-to-digital converter (ADC), an off-chip memory, and on-chip write logic, a buffer, and read logic. The arbitrary waveform generator 102 is composed of an off-chip amplifying and filtering circuit, a digital-to-analog converter, and on-chip reading logic and memory. The clock synchronization module consists of an output buffer outside the chip, a clock buffer, a time sequence generation logic inside the chip and a clock module. The time-to-digital converter 207 is composed of an off-chip comparator, an input buffer, and an on-chip counter, delay chain, encoder, and buffer. The specific working mode of the time-to-digital converter is as follows: the counter is responsible for counting according to the rising edge of the clock signal and is used for recording the coarse time of signal input, and the time resolution is determined by the clock period. The delay chain is composed of a long carry chain structure which is in tail connection, when a signal is input between two clock cycles, the rising edge of the signal propagates in the chain, the output port of each carry chain structure on the chain is changed from 0 to 1 along with the passing of the rising edge in sequence, when the signal is transmitted to a certain position in the delay chain, the rising edge of the clock arrives, the output ports of the carry chains are locked, and the number of the 1 on the output port of the carry chain is calculated to know how many carry chain structures pass in one clock cycle, and the delay of each carry chain structure is approximately equal, so that the specific time of the input signal arriving at the input of the delay chain and the rising edge of the previous clock can be calculated, and the accurate time of a signal arriving can be known by combining the coarse timing result of a counter.

The sweep frequency range of the scanning microwave instrument is 50-70GHz, the working frequency range of the sweep frequency microwave source is 12.5-17.5 GHz, the corresponding control voltage range is 0-20V, the frequency doubling coefficient of the frequency multiplier is 4, the sweep frequency period is 8 microseconds, the sweep frequency repetition frequency is 100KHz, and the design intermediate frequency is 50 MHz. The sampling rate of the arbitrary waveform generator 102 is 250 MSPS. The second data acquisition module 115 has a sampling rate of 250MSPS and an input bandwidth of 125 MHz.

The calibration assembly parameters were as follows: the coupling degree of the directional coupler 201 is-15 dB, the output range of the frequency comprehensive source 202 is 100KHz to 20GHz, the working frequency range of the second mixer 203 is 2-18GHz, the central frequency of the narrow-band filter 204 is 100MHz, the bandwidth is 20MHz, the sampling rate of the second data acquisition module 206 is 1GSPS, and the input bandwidth is 500 MHz. In the time-to-digital converter, the coarse counting clock is 250MHz, the time resolution is 4ns, the average delay of a single carry chain is 40ps, so the minimum time resolution is 40ps, the operating frequency of a microwave switch used in the programmable delay line 208 is DC-40GHz, the insertion loss is 0.4dB @18GHz, the switching time is 15ms, and the lengths of two coaxial cables are 9.0 meters and 0.5 meter respectively. The clock synchronization module 209 is internally provided with a constant temperature crystal oscillator, and the output synchronization clock frequency is 10 MHz.

The following concrete steps are adopted for automatic calibration of the microwave reflectometer:

step one, measuring time delay between a trigger signal and a sweep frequency signal and a beat frequency signal. A pulse waveform with a height of 1V and a width of 50ns is generated by the controller and output to the arbitrary waveform generator 102, from which it is loaded into the memory. The controller 116 instructs the clock synchronization module 209 to generate a trigger signal output to the arbitrary waveform generator 102 and the time-to-digital converter 207. The arbitrary waveform generator 102 outputs the pulse waveform in the memory immediately after receiving the trigger signal. Meanwhile, the time-to-digital converter 207 starts timing with the trigger signal as a start signal in the channel 1, and stops timing with the pulse signal output from the arbitrary waveform generator as a stop signalThen, after averaging the measurements for a plurality of times, the time interval is measured as T_{D_SWEEP}1.240 n. In the channel 2, the time-to-digital converter 207 starts timing by using the trigger signal as a start signal, discriminates the in-phase signal output by the first in-phase quadrature demodulator 112 as a stop signal to stop timing, averages the measurements for multiple times, and measures a time interval T_{D_BEAT}＝42.360ns。

And step two, measuring the time delay between the reference signal and the reflected signal. The controller instructs 116 the programmable delay line 208 to switch from 9.0 meters of coaxial cable to 0.5 meters of coaxial cable, thereby increasing the time delay between the reference signal and the reflected signal. A narrow pulse waveform with a height of 1V and a width of 10ns is generated by the controller 116 and output to the arbitrary waveform generator 102, from which it is loaded into the memory. The controller 116 instructs the clock synchronization module to generate 209 a trigger signal output to the arbitrary waveform generator 102 and the first data acquisition module 115. The arbitrary waveform generator 102 outputs the narrow pulse waveform immediately after receiving the trigger signal, and the first data collection module 115 starts collecting data at the 1GSPS sampling rate. The reference signal firstly reaches the mixer through a delay line, and the reflected signal is transmitted through a waveguide and internally reflected by the device and then reaches the mixer. After mixing, two clusters of waveforms are generated on the beat signal. After the beat frequency signal is collected by the first data collection module 115, the upper envelope of the signal is extracted and peak-finding calculation is performed, and the time interval T between two peak values_{D_TOF_NL}I.e. the time delay between the reference signal and the reflected signal. After averaging over several measurements, T_{D_TOF_NL}60.132 ns. Finally, the controller 116 instructs the programmable delay line 208 to switch from the short coaxial cable back to the long coaxial cable. The difference in cable length is 8.5 meters, and the delay per meter is 4.7ns, resulting in a delay T_LINE39.95ns, i.e. T_{D_TOF}＝60.132-39.950＝20.182ns。

And step three, measuring the dynamic working curve of the microwave source. The controller 116 generates a linear sweep waveform from 0V to 20V, which is transmitted to and stored by the arbitrary waveform generator. The controller 116 instructs the frequency synthesizing source 202 to output a microwave signal at a fixed frequency of 12.625 GHz. Controller 116 command clock synchronization moduleBlock 209 generates a trigger signal output to the arbitrary waveform generator 102 and the second data acquisition module 206. The arbitrary waveform generator 102 receives the trigger signal and immediately outputs a linear scanning waveform, and the second data acquisition module 206 acquires the mixed signal output by the second mixer 203 and then passes through the narrow-band filter 204 and 1 the linear scanning signal generated by the arbitrary waveform generator 02. On the acquired complex signal, two peak values exist in the signal amplitude, the positions of the two peak values are obtained by utilizing a peak searching algorithm, and the scanning voltages at the corresponding moments of the two peak values are 0.110V and 0.923V respectively. Wherein 0.110V corresponds to a frequency 12.625-0.100-12.525 GHz and 0.923V corresponds to a frequency 12.625+ 0.100-12.725 GHz, and thus the midpoint voltage V between the two voltages_SThe frequency 12.625GHz corresponds to (0.110+ 0.923)/2V 0.517V. Therefore, the working voltage is 0.517V when the output frequency of the frequency sweep microwave source is 12.625 GHz. In order to reduce errors, the measurement process needs to be repeated for multiple times, and an average value is taken as a measurement result. The frequency synthesis source 202 is controlled by the controller 116 to convert F_SRepeating the steps according to the increment of the 0.005GHz step value until the measurement of the working voltage in the whole output frequency range of the frequency sweeping microwave source is completed, thereby obtaining the corresponding relation between the working voltage and the output frequency of the frequency sweeping microwave source, namely a dynamic working curve.

And step four, calibrating the beat frequency signal. The controller 116 uses the dynamic working curve of the sweep frequency microwave source obtained in the third step to interpolate and generate a sweep frequency control waveform W with the length of 2000 points according to the equal frequency interval_SWEEPSince the sampling rate of the arbitrary waveform generator is 1GSPS, the sweep period corresponding to the output waveform is 8 us. The controller 116 will W_SWEEPSent to and stored by an arbitrary waveform generator. The controller 116 instructs 209 the clock synchronization module to generate a trigger signal output to the arbitrary waveform generator 102 and the first data acquisition module 115. The arbitrary waveform generator 102 receives the trigger signal and immediately outputs W_SWEEPAnd (6) outputting. The first data acquisition module 115 starts to acquire the frequency sweep control voltage signal output by the arbitrary waveform generator 102 and the beat signal output by the in-phase quadrature demodulator 112 immediately after receiving the trigger signal, and the acquisition time duration is 10 us. Controlling the collected frequency sweepThe voltage signal is denoted as S_SWEEPAnd the collected beat frequency signal is recorded as S_BEATThe number of sampling points was 2500. Data alignment is performed below, since T_{D_SWEEP}1.240ns is less than the sampling period 4ns, so S_SWEEPThe front-end data does not need to be processed, only needs to be processed by S_SWEEPMiddle front T_SWEEP*F_ADCCutting 8us 250MSPS 2000 points, and recording the cut data as S_{SWEEP_S}All information of the sweep control voltage is stored in the data. To align with the swept control voltage signal, S_BEATMiddle front T_{D_BEAT}*F_ADC42.360ns 250MSPS 10 data are truncated, then in the rest of the signal, the first 2000 points are truncated, and the truncated data is marked as S_{BEAT_S}. To S_{BEAT_S}Performing fast Fourier transform, windowing, window width 32 and step length 1 to obtain time frequency spectrum FS_{BEAT_S}. Will S_{SWEEP_S}Copy one copy, denoted S_{SWEEP_FIX}And the frequency sweep control waveform is used for storing the corrected frequency sweep control waveform. Starting from the 2000 th data, for FS_{BEAT_S}(2000) Performing peak searching to obtain the maximum component in the frequency spectrum corresponding to the frequency F_B(2000) 48.325MHz is the beat frequency. The beat frequency is actually mixed from the reference signal and the reflected signal at the time of 2000, and the frequency of the reference signal at 2000 is controlled by the sweep control voltage S_{SWEEP_S}(2000) Determining that the frequency of the reflected signal is Δ m before the 2000 th point as T_{D_TOF}*F_ADCSweep control voltage S of 5 points_{SWEEP_S}(1995) And (6) determining. Since the reflected signal is transmitted through the waveguide to generate dispersion, and therefore frequency drift occurs, we need to modify S_{SWEEP_S}(1995) To correct the beat frequency. The specific correction process is as follows. The voltage S will now be controlled according to the corrected frequency sweep_{SWEEP_FIX}(2000) And a microwave source frequency sweep control curve, and the ideal frequency of the reference signal at the 2000 point can be calculated to be F_{P_LO_FIX}(2000) 17.5GHz, combined with the ideal beat frequency of 50MHz, the ideal frequency at which the reflected signal can be obtained should be F_{P_RF_FIX}(2000) 17.5GHz-0.050GHz 17.450 GHz. Controlling the voltage S according to the frequency sweep_SWEEP(2000) Andthe actual frequency of the reference signal at the 2000 point can be calculated to be F by a frequency sweep control curve of the microwave source_{P_LO}(2000) 17.5GHz, combined with the actual beat frequency of 48.325MHz, the actual frequency of the reflected signal is F_{P_RF}(M) ═ 17.5GHz-0.048325GHz ═ 17.451675 GHz. The difference between the ideal frequency and the actual frequency of the reflected signal is F_{P_RF_FIX}(2000)-F_{P_RF}(2000) 17.450GHz-17.451675 GHz-0.001675 GHz. Since the difference is via T_{D_TOF}Cumulatively, the correction frequency is therefore spread over 5 sweep points, and the correction coefficient η is 1+ [ F [ ]_{P_RF_FIX}(M)-F_{P_RF}(M)]/(F_{P_RF}(M) Δ M) ═ 1-0.001675GHz/(17.451675GHz 5) ═ 0.99998. Controlling the voltage S according to the frequency sweep_{SWEEP_S}(1995) And a microwave source frequency sweep control curve to obtain the original detection frequency F_P(1995) Multiplying the frequency by a correction coefficient eta to obtain a corrected frequency F_{P_FIX}(1995)＝0.99998F_P(1995) Inquiring the sweep frequency control curve of the microwave source again to obtain F_{P_FIX}(1995) Converted to voltage value and filled with corrected sweep control voltage S_{SWEEP_FIX}(1995). By this point, beat frequency correction at 2000 th point is completed, and so on, and 1999, 1998, to 6 th point, one by one. This is done at 6 points because at the 6 th point, the actual correction is S_{SWEEP_FIX}(1) And the correction is finished. It is worth mentioning that when correcting the 1995 th point, S used to calculate the ideal frequency of the reference signal at this time_{SWEEP_FIX}(1995) Modifications have been made at 2000, where modified data was used to modify 1995 instead of unmodified data, which speeds up the modification. Finally, the controller 116 will obtain S_{SWEEP_FIX}The arbitrary waveform generator 102 is written to and stored by it. Because the possible effect of single calibration can not meet the requirement, the step needs to be repeated for multiple times until the beat frequency error of each detection point is less than 1%. All correction steps are now complete.

The above description is only for the specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto, and these examples are only for illustrative purpose and are not intended to limit the scope of the present invention. The scope of the invention is defined by the appended claims and equivalents thereof. Various alternatives and modifications can be devised by those skilled in the art without departing from the scope of the invention, and these alternatives and modifications are intended to fall within the scope of the invention.

Claims

1. A microwave reflectometer with an online automatic calibration function comprises a microwave reflectometer body, wherein the microwave reflectometer body comprises a sweep frequency microwave source (101), an arbitrary waveform generator (102), a first directional coupler (103), a local vibration source (104), a single-sideband modulator (105), a power divider (106), a first mixer (111), a first in-phase quadrature demodulator (112), a first data acquisition module (115) and a controller (116), wherein the sweep frequency microwave source (101) generates a sweep frequency microwave signal under the control of a sweep frequency control voltage signal generated by the arbitrary waveform generator (102), the local vibration source (104) generates a baseband signal with fixed frequency, the power divider (106) divides the baseband signal into multiple paths for output, the sweep frequency microwave signal is divided into a detection signal and a reference signal by the first directional coupler (103), the single-sideband modulator (105) synthesizes the baseband signal and the detection signal to generate an upper sideband signal, the upper sideband signal is transmitted to the plasma, and the receiving antenna (114) receives a reflected signal reflected back from the plasma cut-off layer; a first mixer (111) mixes the reference signal with a reflected signal to obtain an intermediate frequency signal, and a first in-phase quadrature demodulator (112) complex mixes the intermediate frequency signal with a baseband signal to obtain a beat signal, characterized in that:

the calibration assembly comprises a second directional coupler (201), a frequency synthesis source (202), a second mixer (203), a second data acquisition module (206), a time-to-digital converter (207) and a clock synchronization module (209), wherein the second directional coupler (201) separates a part of a frequency-sweeping microwave signal output by the frequency-sweeping microwave source (101) to be used as a calibration signal, the frequency synthesis source (202) generates a microwave signal with a fixed frequency, the second mixer (203) is used for mixing the calibration signal and the microwave signal with the fixed frequency to generate a difference frequency signal of the calibration signal and the microwave signal, the second data acquisition module (206) is configured to obtain a dynamic working curve formed by the working voltage of the frequency-sweeping microwave source (101) based on the difference frequency signal, and the clock synchronization module (209) is controlled by the controller to generate a trigger signal, the time-to-digital converter (207) is configured to measure a first time delay between the trigger signal and the swept frequency microwave signal and a second time delay between the trigger signal and the beat frequency signal, the first data acquisition module (115) is configured to measure a third time delay between the reference signal and the reflected signal, and the controller (116) is configured to be connected to the arbitrary waveform generator (102) and to modify the beat frequency signal by using the dynamic operating curve and by modifying the sweep frequency control voltage output by the arbitrary waveform generator (102) based on the first, second and third time delays.

2. The microwave reflectometer with on-line automatic calibration function as in claim 1, wherein: the frequency sweep microwave source (101) generates the frequency sweep microwave signal with the frequency changing along with the voltage under the control of the frequency sweep control voltage signal generated by the arbitrary waveform generator (102).

3. The microwave reflectometer with on-line automatic calibration function as in claim 1, wherein: the microwave reflectometer body further comprises a first frequency multiplier (107), a second frequency multiplier (108) and a third frequency multiplier (109), wherein the first frequency multiplier (107) is used for multiplying the frequency of an upper sideband signal, the second frequency multiplier (108) is used for multiplying the frequency of a reference signal, the third frequency multiplier (109) is used for multiplying the frequency of a baseband signal, the frequency-multiplied upper sideband signal is transmitted to plasma, the reference signal output by the second frequency multiplier and a reflection signal are mixed by the first frequency mixer (111), and an intermediate frequency signal and the baseband signal output by the third frequency multiplier are subjected to complex mixing by the first in-phase quadrature demodulator (112).

4. The microwave reflectometer with on-line automatic calibration function as in claim 1, wherein: the calibration component further comprises a programmable delay line (208), the programmable delay line (208) is used for adjusting the delay time of the reference signal output by the first directional coupler (103), and the two microwave switches are switched to realize the switching of coaxial cables with different lengths.

5. The microwave reflectometer with on-line automatic calibration function as in claim 1, wherein: the beat frequency signal has an in-phase component and a quadrature component, and the first data acquisition module (115) acquires the in-phase component, the quadrature component and the sweep frequency control voltage signal and outputs the same to the controller (116).

6. The microwave reflectometer with on-line automatic calibration function as in claim 1, wherein: the calibration component further comprises a narrow-band filter (204), a second in-phase and quadrature demodulator (205), the second mixer (203) outputs to the narrow-band filter (204), the center frequency of the pass band of the narrow-band filter (204) is equal to the frequency generated by the local oscillator (104), the second in-phase and quadrature demodulator (205) is used for extracting an in-phase component and a quadrature component of the output signal of the narrow-band filter (204), and the second data acquisition module (206) is used for digitizing the signal output by the second in-phase and quadrature demodulator (205) and transmitting the digitized signal to the controller (116).

7. The microwave reflectometer with on-line automatic calibration function as in claim 1, wherein: the core component of the sweep frequency microwave source (101) is a voltage controlled oscillator.

8. The microwave reflectometer with on-line automatic calibration function as in claim 1, wherein: the second data acquisition module (206), the time-to-digital converter (207), the clock synchronization module (209) and the arbitrary waveform generator (102) are integrated on a printed circuit board.